1. Field of the Invention
The present invention relates to a thermal activation method and processing method for a heat-sensitive adhesive sheet, a thermal activation device, and a printer for a heat-sensitive adhesive sheet.
2. Description of the Related Art
Heat-sensitive adhesive sheets with a heat-sensitive adhesive layer that develops adhesion when heated, as those disclosed in JP 11-79152 A and JP 2003-316265 A, have been in practical use for some time now. Such heat-sensitive adhesive sheets have advantages including being easy to handle since the sheets are not adhesive prior to heating and producing no factory wastes since they do not need release paper. A thermal head, which is usually employed as a printing head in a thermal printer, is sometimes used to heat this type of heat-sensitive adhesive sheet and to thereby make its heat-sensitive adhesive layer develop adhesion. This is advantageous particularly when a heat-sensitive adhesive sheet is printable on one side, for thermal heads similar in structure can be used for printing and thermal activation. A common thermal activation device has a thermal head as the one mentioned above, and a platen roller which rotates against the thermal head. A heat-sensitive adhesive sheet is inserted between the thermal head and the platen roller and, during the passage, a heat-sensitive adhesive layer of the sheet is thermally activated from the heat of the thermal head, thus developing adhesion.
In general, when attaching an adhesive sheet to some article, the adhesive sheet, particularly its outer edges, should be stuck solid to the article. As long as the outer edges of the adhesive sheet are glued fast, a loose portion in the middle hardly causes the adhesive sheet to fall off in use and raises no substantial problem. On the other hand, if the outer edges of the adhesive sheet are loose in some places, the adhesive sheet can easily start to peel from those places, which seriously damages the function of the adhesive sheet as well as the reliability of a device or the like that has attached the adhesive sheet.
A conventional solution to this problem is, in the case where a heat-sensitive adhesive sheet is to be thermally activated and develop adhesion from contact with a thermal head which is generating heat, to start driving the thermal head and thereby make the thermal head generate heat before the heat-sensitive adhesive sheet enters between the thermal head and an opposing platen roller, and to stop driving the thermal head and thereby make the thermal head cease generating heat after the heat-sensitive adhesive sheet departs from between the thermal head and the platen roller. This is to accommodate errors of a heat-sensitive adhesive sheet conveying device and of other relevant devices which could cause a heat-sensitive adhesive sheet to move in other manners than intended and fail to enter, or depart from, between the thermal head and the platen roller at a given timing. In other words, this avoids insufficient thermal activation of the front end or rear end of a heat-sensitive adhesive sheet in a sheet conveying direction due to ill-timed heat generation of the thermal head, including cases where the thermal head is not ready to heat the front end upon its arrival at the passage between the thermal head and the platen roller, and cases where the thermal head stops generating heat prior to departure of the rear end from between the thermal head and the platen roller, as well as resultant spots of weak adhesion in the front end or rear end of the heat-sensitive adhesive sheet. Similar drive control to ensure that the front and rear ends of a heat-sensitive adhesive sheet are heated well is executed in the case where the thermal head does not generate enough heat immediately after started up and accordingly needs pre-heating time.
Thus, driving the thermal head longer than a given period in which a heat-sensitive adhesive sheet enters and departs from between the thermal head and the platen roller makes it possible to obtain a highly reliable heat-sensitive adhesive sheet with good adhesion irrespective of some error in conveyance of the heat-sensitive adhesive sheet.
As described, a heat-sensitive adhesive sheet with good adhesion is obtained by the thermal activation method that keeps the thermal head driven longer than a given period in which a heat-sensitive adhesive sheet enters and departs from between the thermal head and the platen roller. A drawback of this method is that, if the heat-sensitive adhesive sheet has a heat-sensitive printable layer besides the heat-sensitive adhesive layer, the printable layer may develop color unintendedly (blurring) from excess heat.
To elaborate, while the thermal head is driven to generate heat prior to arrival of the heat-sensitive adhesive sheet at the passage between the thermal head and the platen roller and while the thermal head remains driven to generate heat after departure of the heat-sensitive adhesive sheet from the passage, the platen roller rotates pressed directly against the thermal head instead of through the heat-sensitive adhesive sheet. Directly heated by the thermal head, the platen roller accumulates heat. When the heat-sensitive adhesive sheet is inserted between the thermal head and the platen roller that has accumulated the heat, the printable layer is heated and develops color through contact with the platen roller and from the accumulated heat on the surface of the platen roller at the same time the heat-sensitive adhesive layer is heated through contact with the thermal head. The surface of the platen roller is often formed from a highly heat-resistant material such as silicone rubber and, because of the low thermal conductance of the material, can keep heat well enough to cause the printable layer to develop color unintendedly.
As has been described, in prior art, a measure to ensure satisfactory thermal activation of the front end of a heat-sensitive adhesive sheet in a sheet conveying direction could result in unintended color development of a printable layer of the heat-sensitive adhesive sheet and a measure to ensure satisfactory thermal activation of the rear end of a heat-sensitive adhesive sheet in a sheet conveying direction could result in unintended color development of a printable layer of the next heat-sensitive adhesive sheet.